This invention relates to semiconductor transistors, and, more particularly, to gate electrodes for field effect transistors.
Semiconductor transistors are found in nearly all electronic devices. They can be made in miniature forms useful in integrated circuits and other electronic packages of reduced size. The introduction of the transistor and its continuing development in ever-smaller forms is one of the major factors in the growth in popularity of consumer devices such as hand-held calculators and personal computers, and in sophisticated business and military electronic systems.
The operation of small-scale transistors is generally understood from the point of theory, but it remains to develop the fabrication techniques for constructing the devices reproducibly and reliably, on a commercial basis. As the size of the transistors is reduced, many of the fabrication techniques routinely employed in preparing larger devices cannot be used because they reach some inherent physical limitation. For example, visible light photoresist techniques used to fabricate larger transistors cannot be used for devices of less than a micrometer in size due to limitations imposed by optical effects. Well known fabrication techniques therefore may become obsolete as further size reductions are achieved. The effort to achieve further miniaturization is therefore closely linked to the development of processes for preparing the desired structures.
One important type of transistor is the field effect transistor, of which there are a number of varieties. The metal-semiconductor field effect transistor, or MESFET, is a voltage controlled variable resistor. A controllable electrical current can be established between source and drain electrodes of the MESFET, with the current controlled by a voltage applied to a gate electrode that is positioned on the semiconductor substrate between the source and drain electrodes.
Field effect transistors can be fabricated from many types of semiconductor materials. Gallium arsenide is a semiconductor having excellent properties for use at high frequencies. Gallium arsenide MESFETs are of particular interest for use in electronic circuits for low-noise amplification of signals, high-efficiency power generation, and high-speed logic devices, among others. Investigations directed at reducing the size of gallium arsenide MESFETs for use in integrated circuits and other applications are ongoing.
The performance of MESFETs is determined in part by the dimensions, electrical resistance, and capacitance of the gate electrode. Higher resistance and capacitance of the gate electrode are undesirable, because they reduce the high frequency performance of the MESFET in circuits. As the length of the contact surface or gate of the electrode parallel to the direction of current flow is reduced, the resistance of the gate electrode increases and its capacitance decreases. That is, as the gate length is made shorter, its resistance rises and becomes the dominant factor in limiting the operating frequency of the MESFET. As the size of MESFETs is reduced, the necessary reduction in the size of the gate can therefore limit its performance and effectively prevent further reduction in size.
Various geometrical arrangements and related techniques for preparing the gate electrode have been proposed, with the result that the gate length has been reduced to about 2500 Angstroms in the current art. Gates of this length can be fabricated reasonably reliably, and the resulting devices have acceptable performance. It is, of course, of interest to reduce the size of the field effect transistor, the gate, and the gate electrode even further, to permit even higher frequency operation and greater density of devices in integrated circuits. To achieve further miniaturization, it is necessary to reduce the gate length without unduly increasing the gate electrode resistance, but no approach and fabrication process for achieving this result, where the fabrication process can achieve acceptably high device yields, has been proposed.
Accordingly, there exists a need for a gate electrode geometry and fabrication techinque that permits the commercial-scale fabrication of smaller gates for the use in field effect transistors and possibly other electronic devices. The present invention fulfills this need, and further provides related advantages.